Development of new chromosomal differential staining techniques and continued improvement of established methods are basic to all of the projects conducted in the laboratory. During the past year, two more precise methods for studying chromosome structure and function were introduced: sister chromatid exchange, a sensitive means of visualizing chromosomal damage and repair, and analysis of chromosomal replication sequences by differential staining, a technique which replaces autoradiography. These improved methods were then utilized for all major studies in progress. Human cytogenetic studies of patients with congenital and developmental disorders as well as studies of mutagen-induced chromosomal abnormalities are continuing. In addition, differential staining methods are now being used to study patients exposed to benzene and related polycylic hydrocarbons. Murine cytogenetic studies include evaluation of mitotic and meiotic chromosomes of Robertsonian translocation carriers and karyotypic evaluation of cells in hematopoietic tissue regeneration experiments. These are part of long term projects utilizing the mouse as a model for human disorders.